Boreholes are drilled into the earth for many applications such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. In order to efficiently use expensive resources drilling the boreholes, it is important for analysts to acquire detailed information related to the geologic formations being drilled.
Nuclear magnetic resonance (NMR) tools are one type of downhole tools that are particularly useful for performing detailed measurements of properties of hydrocarbon bearing formations or overburden shale. NMR measurements are used to determine among other things, porosity, hydrocarbon saturation, and permeability of rock formations. In overburden shale, the porosity may be due to clay-bound water since there may be is no hydrocarbon in this shale. The NMR logging tools are used to excite the atomic nuclei of the fluids in the geological formations surrounding the borehole so that certain NMR parameters such as NMR porosity, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to in the art as T2) of the geological formations can be measured. From such measurements, the porosity, permeability and hydrocarbon saturation are determined, which provide valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons. The following references may be referred to for teachings with respect to performing NMR measurements: NMR LOGGING PRINCIPLES & APPLICATIONS by George R. Coates, Lizhi Xiao, and Manfred G. Prammer, Halliburton Energy Services Publication H02308 (1999); Nuclear Magnetic Resonance Petrophysical and Logging Applications by K.-J. DUNN, D. J. Bergman and G. A. Latorraca, PERGAMON (2002); and U.S. Pat. No. 6,051,973 to Manfred Prammer.
Determining in particular fast decaying partial porosities at downhole conditions has been challenging. Prammer et al. (Prammer et al., SPE Annual Technical Conference and Exhibition, Denver 6-9 Oct. 1996, SPE-36522) describes a method to separately determine fast decaying partial porosities by performing two NMR measurements with a short and a long wait time. Prammer et al. records two echo trains and estimates a fast and slowly decaying porosity. Akkurt et al. (Akkurt et al. SPWLA 39th Annual Logging Symposium, Keystone, Colo., 26-28 May 1998, SPWLA-GG) describes a so-called “dual-TE method” to determine the fluid diffusion coefficient by utilizing two NMR measurements with equal wait time and different inter-echo times. The wait time for the two NMR measurements is long in order to polarize all of the formation fluid. The fluid diffusion coefficient is estimated from the two echo trains that are recorded with the two NMR measurements.
Unfortunately, an NMR effect, here referred to as the second-order stimulated-echo effect, disturbs the amplitude of indirect NMR echoes. The theoretical background for this effect was presented by Goelman and Prammer (Goelman, G. and Prammer, M. G. 1995, The CPMG Pulse Sequence in Strong Magnetic Field Gradients with Applications to Oil-Well Logging, Journal of Magnetic Resonance, Series A 113: 11-18). However Goelman and Prammer do not mention the consequence of the second-order stimulated echo effect on the porosity in geological formations with micro-porosity. The disturbance leads to distortion of an NMR signal and is significant in formations with NMR micro-porosity (e.g., shale gas, shale oil, clay-bound water, heavy oil, tar, carbonates). Due to the second-order stimulated-echo effect prior art NMR logging methods can lead to inaccurate estimates of micro-porosities where the estimates can be inaccurate by 20% or even more. Hence, it would be well received in the drilling and production industries, if methods and systems could be developed to reduce or eliminate the distortion of the NMR signal due to the second-order stimulated-echo effect.